1. Field of the Invention
This invention pertains to the treatment of sludge and particularly to techniques for processing relatively large volumes of sludge initially containing a large percentage of liquid so as to quickly and efficiently remove a sufficiently large quantity of the liquid from the sludge to make the mostly liquid-free residue readily transportable.
2. Description of the Prior Art
In nearly every industrial and municipal wastewater treatment process, the need to thicken or consolidate waste solids in order to reduce their volume for economical disposal is one of ever increasing proportions. The need parallels the demands of a growing population for increased treatment capacity and better effluent quality.
Because waste solids, consolidated or otherwise, offer little potential for monetary return in the marketplace, great emphasis is placed on minimizing installation and operating costs of the necessary mechanical consolidation elements.
The art of solids-liquid separation finds application in thickening or consolidation of waste solids. In general though not all inclusive sense, this art is an expression of two technologies: chemical and mechanical.
Chemical contributions have been extensive. For centuries the utilitarian benefits of inorganic metallic salts or coagulants and flocculants have been known. Because of a lacking structural integrity of agglomerated particulate matter formed through their use, the physical methods employed to effect separations were limited in their effectiveness. The availability of high molecular weight organic polyelectrolytes (within the past 30 years) has been a beneficial development of genuine merit. The strength or integrity of agglomerated particulate matter developed through their use now permits the successful utilization of a variety of mechanical devices to assist in th physical aspects of separation. An understanding of the utility of high molecular weight polyelectrolytes can be appreciated from an appraisal of a typical waste sludge that is a thin, watery suspension of micro fine particulates, each carrying an electrical charge. Such particles, if under the influence of an electrical field and negatively charged, will migrate or be attracted to the cathode or positively charged electrode.
In the sludge, however, if there is nothing present to supply cations or positive charges and the sludge is anionic in general character, the particles try to stay independent and apart from each other. This system of repulsion forces may be compared to two magnets with like poles in proximity, repelling or pushing apart each from the other.
The infinite number of micro particles and the multiplicity of like and repelling charges sets up what is essentially a stable system of repulsion forces that prohibits significant natural thickening or consolidation beyond the range of 1-3% solids. Each sludge exhibits such behavior and it may reflect a preponderance of positive or negatively charged character depending on the program yielding the sludge.
The system of repulsion forces may be destabilized or in a sense neutralized by the mixing in of an additive that exhibits a character opposite to that reflected in the sludge. If, for example, the sludge is anionic in character, the use of a cationic additive is indicated and the quantity required is determined by the extent of the demand for th additive.
In a destabilized system or where, for example, a cationic demand has been neutralized with the use of an additive, the micro particles no longer repel each other and may adhere to each other on contact to form groups, with the groups so formed adhering to other groups to form larger groups to become visibly identifiable agglomerated matter referred to as "flocs". In addition to charge neutralization, the high molecular weight additives help provide strength to the developed flocs through a mechanism commonly referred to as "bridging". Thus, charge neutralization and bridging are believed to be the mechanisms through which flocculation can be made to occur.
Once the particulate matter in a sludge suspension has been properly neutralized and agglomerated through the means previously described, the usefulness of mechanical separation equipment becomes apparent.
All known processes that functionally assist solids-liquid separation operate using gravity, pressure, vacuum, centrifugal force, or a combination of these forces at either ambient or elevated temperatures.
Not necessarily in the order of their development, the list of known devices contributing to the artful science of solids-liquid separation include: batch type and continuous centrifuges, continuous vacuum filters of the rotary drum type, straight line traveling belt type, plate and frame batch type presses, rotary and straight line traveling belt draining devices, continuous belt presses, batch character sand bed filtration and drying beds, and more recently, vacuum assisted units of the same character. Also known are presses of the screw type and combinations of the various devices listed.
The processes listed have served usefully in the mechanics of separation. These and other contributions reflect progress in the areas of filtration, centrifugation, vacuum and pressure development.
However, it is apparent from the economics that it is highly desirable to accomplish more with each dollar invested in wastewater plants and process equipment. Government financial assistance is on the decline while demands for better effluent quality and increased throughput are constantly growing. Logically, the accomplishment of higher throughputs in any segment of plant operations benefits the entire operation. To do this in the solids-liquid separation area, on the basis of a low-to-negligible additional investment, must be well regarded as a beneficial improvement. The impact can be a positive one from one end of the plant to the other as bacterial population control or solids management is a key element of efficient operation. The system described herein is comprised of several parts, each of which includes novel features and which together provide a novel system.
The weaknesses or shortcomings of existing processes and equipment may be summarized as follows:
1. Preoccupation with the element of dryness:
Consider 100,000 gallons of 1% sludge. As is, this represents 833,000 pounds of material requiring disposal in order to remove the waste solids portion of 8,330 pounds of dry material. This means that 824,670 pounds of water must be disposed of in order to remove the 8,330 pounds of sludge. The water portion of this problem is 99%.
Assume, for purposes of understanding, what the situation looks like if the sludge is converted to 20% material--a figure that is desirable in terms of dryness but one which in reality, is seldom, if ever, reached in actual continuous process equipment.
The disposal problem has been reduced from 833,000 pounds to 41,650 pounds. The dry solids portion is still 8,330 pounds. 791,350 pounds of water are no longer part of the disposal problem. It is simply returned as harmless water to the plant circuitry. 95% of the free water has been removed.
If, on the other hand, 15% solids is the figure reached through consolidation, the disposal problem has been reduced from 833,000 pounds to 55,533 pounds, leaving 777,467 pounds of water for return to the plant. This means that 93.3% of the free water has been removed.
The difference is a relative insignificant 1.7% of the available free water and the importance of getting this small extra percentage has been vastly overemphasized. The cost to accomplish this is, and has been, exorbitant. For example, a continuous belt press, sized with a 1.5 meter belt width will be capable of processing approximately 70 gpm of sludge, and may, under good operating conditions, produce a cake with 20% solids. Of the continuous processes available today, the belt presses are the only units that approach 20% output solids. With the auxiliary equipment required to make it operational, such a unit as has just been described, will represent a capital investment exceeding $200,000. This figure represents a cost of $2,857 per gallon of throughput capacity. Suppose then, because of a new approach, it were possible to push 300 gallons per minute through a unit that costs the same $200,000 but was only capable of reaching 15% solids. This would reduce the cost per gallon throughput to $666. It would seem that the 1.7% extra water removed would truly take on the character of an unimportant matter in view of the saving. This is what can be accomplished with technique described herein when used in conjunction with finishing pressure elements.
2. Throughput limitations:
This topic is interrelated to topic 1 above. The system described herein increases throughput compared to prior art systems, making one unit serve the purpose of several. Moreover, the system herein yields cost effectiveness and reduces cost of operation because each man hour serves more usefully and chemical usage has been minimized.
3. Application inflexibility:
Trying to get existing equipment into a specific thickening mode of operation is very difficult, that is, where the output can be controlled in the 5-7% solids range. The apparatus described herein, however, is designed to serve efficiently in this output solids range in addition to the dewatering solids range of 14-15%, when used in conjunction with finishing pressure elements.
4. Operational complexity:
Much of the equipment on the market today is entirely too complicated. That is, it is too complicated for the average operator and too complicated and time consuming in repairs. The apparatus described herein is so simple, so easy to operate, so simple to maintain and repair, that it will set a new standard for simplicity in such matters.
5. High utility requirements:
The requirements for water and power of the known processes exceed 60 amps and 50 gallons per minute, respectively. The system described below utilizes approximately 30 amps for power and 8 gallons per minute for water.
Hence, it is a primary feature of this invention to focus attention on the peculiar properties of those chemical additives known as polymers or high molecular weight polyelectrolytes, the use of which made accelerated solids-liquid separations possible, and to provide a process that uniquely takes advantage of these properties.
It is another feature of this invention to provide an improved process, consistent with the above, that will result in lower operating costs, higher throughputs, operational simplicity, and lower utility requirements when compared to other processes known at this time.
It is a further feature of this invention to provide applications flexibility; that is, to provide simple adaptability to what is commonly known as a "thickening"
operation where output solids are expected to be in the 5-7% range, or a "dewatering" operation where output solids are expected to be in the 15% range, when used in conjunction with finishing pressure elements such as auger presses, centrifuges and/or belt presses.
Especially with regard to the first feature set forth above, lip service appears to be the extent of attention to date to the process of "mixing". Numerous references are made to this subject in earlier patents with such profound comments as "mixing shall be by suitable means" without providing any parameter or guidelines to such mixing.
Mixing of a sludge with a coagulant or flocculant, to the contrary, is the basis of a successful separation experience. The benefits of attention to the intensity and mechanical aspects of mixing can result in a number of operational dividends. In a controlled mixing environment, even to an untrained observer, it is simple to determine when the proper amount of chemical has been added. Consequently, it is easily determined when too much has been added, and excessive chemical usage can be avoided.
Excessive chemical usage should be avoided for two reasons. First, it is expensive and bears heavily on the overall cost of solids-liquid separation. In every plant, this is a persistent target in efforts to economize. Secondly, an overuse of chemical hinders the drainage rate and is counterproductive to the purpose of the chemical use.
Shear, or destructive forces should be avoided if at all possible. If they are not avoided, these forces will service to destroy or harm or break down the floc-like agglomerates once formed, the formation of which was the intended purpose in the first place.
Anything like a propeller running at high speed, a baffle against which the agglomerated matter might crash, sudden directional changes in flow, vertical and at the same time horizontal impetus all serve to simply frustrate the intended use of the chemical. The purpose of purely mixing may be served by such means to be sure, but always an extra price will be extracted in the form of extra chemical to toughen the agglomerates and make them more resistant to the effects of such abuse.
It is pure, indisputable logic in the pursuit of both efficiency and economy, therefore, to address mixing as the starting matter of dominant importance or the foundation on which everything must depend. This may be demonstrated by considering the abusive extreme of mixing chemical and sludge in a blender. They certainly would be "mixed" everyone would agree. But, if the purpose of the mixing is separation of the contained solids, the purpose would be completely frustrated. Nearly every manufacturer of equipment on the market today considers this matter lightly, as of no major importance, requiring attention in a secondary fashion or a bridge to be crossed when it becomes a problem.